The invention concerns a process for heat treating parts comprising multiple dissimilar metals and/or alloys to simultaneously soften some metals and/or alloys and harden others, and parts made by the method. More particularly, the invention concerns heat treating a bullet having a core and a jacket on at least a portion of an external surface of the core, wherein the heat treatment simultaneously hardens the core and softens the jacket.
A typical cartridge includes a casing and a bullet. The casing is a generally cylindrical container that houses powder and incorporates a primer on one end and an opening on the other end. A bullet is fitted into the open end. A firing pin strikes and detonates the primer to ignite the powder, which produces a large gas pressure that forces the bullet outwardly and away from the casing, down and out of the firearm""s barrel.
Bullets are known that have an inner core and an outer jacket on at least some portion of the core""s outer surface. The inner core usually is made from a high-density, high-ductility, low-cost material, such as lead. The outer jacket is made of a harder material, such as copper. Bullets having this structure are referred to herein as ball ammunition bullets.
Bullets are jacketed for several reasons. First, lead and most lead alloys have yield strengths of 10,000 psi or less. Pressures produced within the casing exceed these yield strengths, and can be at least as high as 50,000 psi. Such high pressures can deform nonjacketed lead bullets and thereby deleteriously affect the projectile""s performance. Second, the temperature of the burning powder can be greater than the melting point of lead and lead alloys used to make bullet cores. That portion of a core exposed to a temperature greater than its melting point can melt, and can be deposited on the wall of the barrel. Over time, such deposits can alter the flight of subsequently fired bullets. Jackets can protect the core from damaging temperatures. Third, when the bullet is fired down the barrel, rifling (inner-facing helical grooves within the barrel) forces the bullet to spin. This spin stabilizes and increases the accuracy of a non-jacketed bullet. Lead and lead alloys can be stripped off the unprotected surface of the lead bullet by the rifling. Stripping deposits material in the barrel, which can result in inadequate bullet rotation and, consequently, decreased accuracy. Jackets protect the lead alloy core from rifling damage.
Jackets also increase bullet rigidity and can increase the ability of a bullet to penetrate a given target. A bullet""s penetrating capability predominantly is a function of its impact velocity, the shape of the bullet, and the hardness, strength, and resistance to deformation of a bullet""s component parts. Because both the hardness of the jacket and the core affect the ability of a bullet to penetrate a target, penetrator ammunition bullets have a steel core tip with the remainder of the core being a lead alloy. However, these additional parts included in penetrator ammunition bullets typically decrease their accuracy.
A major factor in a bullet""s trajectory and retained velocity at any given point is the bullet""s ballistic coefficient. The ballistic coefficient is a measure of the bullet""s ability to resist atmospheric frictional drag, and primarily is a function of a bullet""s form and density. A bullet having a greater density will have a better ballistic coefficient, a greater retained velocity at a given point, and a flatter trajectory than bullets that are less dense. Armor-piercing ammunition bullets and penetrator ammunition bullets replace at least part of the lead core with steel, which is less dense than lead or lead alloys, and therefore have poorer ballistic coefficients than otherwise equivalent ball ammunition bullets. Thus, although armor-piercing and penetrator ammunition bullets have greater penetration performance, their in-flight performance is poor when compared to denser ball ammunition bullets.
Knowing the effects of core hardness on a bullet""s penetrating capability, conventional methods known prior to the present invention heat-treat bullets without applying a jacket to the core. Precipitation hardening is one heat-treating process that has been used to harden bullets made from lead alloys. Precipitation-hardening has four steps: first, the metal is heated to a sufficient temperature; second, the metal is held at this temperature for a period of time (commonly referred to as xe2x80x9csoak timexe2x80x9d); third, the metal is quenched in a liquid; and fourth, the metal is aged, which refers to the period subsequent to quenching and prior to any further mechanical or heat processing, and during which the physical property in question is changing, e.g., during which period the hardness of the bullet increases. The metal may seem softer than it was originally after the heating and quenching process but the hardness increases as the metal ages. For a description of precipitation-hardening and a discussion of possible explanations for the resulting increase in hardness, see William Howard Clapp and Donald Sherman Clark, Engineering Materials and Processes 183-84 (1954).
The hardness of a bullet typically is measured by a Brinell Hardness Number (BHN). This value is useful because it is directly proportional to the yield strength of the metal tested. A BHN value for an article is obtained using a test device. The test device includes a spring-loaded plunger that screws into a loading press and applies a known load on a ball bearing, which then creates a small crater in the object being measured. If the dimensions of all components in the test device are known, the BHN is determined by the equation:
BHN=0.0004485*F/{(xcfx80/2)*D2*[1xe2x88x92(1xe2x88x92(d/D))2]}
where
F=load, pounds
D=ball diameter, inches
d=diameter of crater in sample, inches
xcfx80=3.14159
BHN values of lead cores produced by conventional bullet processing methods are typically reported in units of kg/mm2 (such values also can be stated simply as Brinell Hardness Numbers, e.g., BHN values of 8 kg/mm2 are reported as 8 or 8 Brinell). From the BHN, the core yield strength of the measured object is then determined by multiplying the BHN by about 515. For a review of how to construct a test device to measure the Brinell Hardness Number and core yield strength, see Harold R. Vaughn""s Rifle Accuracy Facts, Precision Shooting, Inc. Press, Manchester, Conn., (1998).
Prior heat-treated bullets typically have been mechanically worked after the core heat treatment to resize and shape the bullet and apply the jacket to the core. Mechanical working or re-heating steps realign the grains of the core, allow slipping within the metal, and thus decrease the original core hardness obtained by heat-treating.
Another problem that has been encountered in maximizing the penetrating capability of bullets has been the brittle nature of most jackets. During the process of applying the jacket to the bullet and resizing or reshaping the bullet, the jacket becomes brittle as the orientation, shape, and/or size of the grains are altered. Because the jacket is brittle, it tends to crack and fractures quickly upon impact, which decreases its penetrating capability.
The decrease in a bullet""s penetration capability due to lead cores that are not sufficiently hard and jackets that are too brittle may be offset to some extent by increasing the jacket thickness. A thick jacket is less prone to fracture easily upon impact and is more likely to offset the problems of a core that is too soft. However, jacket materials, such as copper and copper alloys, typically are much more expensive than the core materials, such as lead and lead alloys. Both manufacturing time and expense are increased if a thicker jacket must be applied to the core.
An even more significant problem associated with known lead-core jacketed bullets is the final bullet rigidity which is a product of both the core rigidity and the jacket rigidity. A convenient rule of thumb is that every 0.001 inch copper jacket added to a core increases the effective bullet hardness by about 1 Brinell Hardness Number. Bullets produced by methods known prior to the present invention have included copper or copper-alloy jackets as thick as 0.030 inch, although a typical jacket thickness is less than 0.008 inch. The Brinell hardness of lead cores produced by conventional bullet processing methods typically is about 8-9 Brinell. As a result, the rigidity index (core hardness plus jacket thicknessxc3x971,000 inches) of prior bullets typically are significantly less than 20, and more likely are from about 15 to about 17. Methods for increasing bullet rigidity while decreasing jacket thickness and production costs are therefore still needed.
The present invention addresses the problems discussed in the Background. One embodiment of the present invention for manufacturing parts in general comprises: (a) forming a first member comprising a first material, such as a nonferrous metal or metal alloy; (b) applying a second material, such as copper or a copper alloy, to an exterior surface of the first member, thereby forming a part; and (c) heating the part at a pre-selected temperature for a period of time sufficient to simultaneously harden the first member, such as through precipitation-hardening, and stress-relieve, and perhaps also anneal, the second material. The process includes quenching the part, such as by immersing it in water or other suitable fluid, after heating the part. The process also can include aging the part after quenching, where aging is continued for a period sufficient to further harden the part.
An example of a part that can be made by the method is a bullet, or other projectile. A working embodiment of the invention comprised a bullet core made from a first material, and a jacket made from a second material that was applied to at least a portion of an external surface of the core. An example of a working embodiment of the core was made from a lead alloy comprising, by weight: (a) from about 0.5 to about 7.4 percent antimony, generally from about 1 percent antimony to about 6 percent antimony, more typically from about 2 percent to about 4 percent antimony, with working embodiments having about 3 percent antimony; (b) from about 0 percent tin to an amount of tin substantially equal to that of antimony; and (c) from about 0 percent to about 0.5 percent arsenic, more typically from about 0.02 percent to about 0.3 percent arsenic, with working embodiments having about 0.10 percent arsenic.
In working embodiments, the second material has been selected from the group consisting of copper, copper alloys, and mixtures thereof. The second material can be applied to the core by a number of methods. A currently preferred method for forming the jacket about at least a portion of the core is electroplating. Another method for applying the jacket to at least a portion of the core includes, without limitation, mechanically applying a jacket formed by conventional cup and drawing processes.
Bullet embodiments having lead-alloy cores and copper or copper-alloy jackets have been heat treated at a temperature within the range of from about 400xc2x0 F. to about 500xc2x0 F., generally from about 450xc2x0 F. to about 480xc2x0 F. In preferred embodiments the part was maintained at a substantially constant temperature within this temperature range for a period of time sufficient to solution treat (harden, generally precipitation-harden) the first material and stress-relieve, and perhaps anneal, the second material. This period of time can vary, but generally has been found to be at least 5 minutes, more typically about 15 minutes, and perhaps even as long as 30 minutes. The duration of the heating period can vary, and depends upon a number of factors including the particular alloy used, the hardness desired in the part and economic reasons for minimizing the heating time.
One example of a jacketed projectile made by the method of the present invention had the precipitation-hardened lead-alloy core described above and a copper or copper alloy electroplated to at least a portion of an external surface of the core to form a jacket having a thickness of about 0.004 inch. Cores produced using this alloy and processed by the method of the present invention had Brinell Hardness Numbers of from about 10 kg/mm2 to about 40 kg/mm2 and core yield strengths of from about 5,150 to about 20,600 psi. These values are appreciably higher than those for cores produced by methods known prior to the present invention, having reported BHN values of 8-9 kg/mm2 and core strengths of 4,120 to 4,635 psi.
The method of the present invention can be used to produce several different bullet types having various Brinell Hardness Numbers for various applications. Solely by way of example and not limitation, the present invention can be used to make jacketed, or at least partially jacketed bullets, such as Blount""s TMJ(copyright) bullets, and soft nose or hollow point bullets, such as Blount""s GOLD DOT(copyright) hollow point bullets.